Plant development and metabolic activity are regulated by several environmental factors. Light is one of the most important environmental stimuli for plant growth and development. Light is perceived by several photoreceptors: far-red and red light by phytochromes (phyA to phyE) and blue and UV-A light by cryptochromes (cry1 and cry2). Whereas cytosolic phytochromes are translocated into the nucleus upon light-mediated activation, cryptochromes are localized in the nucleus (Curr Opin Plant Biol. (2004) 7, 564-9). Significant progress has been made in understanding the functions of photoreceptors and in the identification and characterization of downstream components of light signaling pathways (Science (2000) 288, 859-863; Annu Rev Plant Biol. (2002) 53, 329-55.). Light signaling pathways likely to have strong impact by cross-talk on other signaling pathway for optimum growth and development of plant. Light is perceived by a variety of photoreceptors and transmitted by several downstream signaling components through central regulator that control developmental as well as metabolic processes including photosynthesis and assimilate allocation. It has been very recently shown that productivity of crop plants might be enhanced by overexpressing one of the central regulators of light signaling pathway (Plant Physiology (1999) 120:73-81; Proc Natl Acad Sci USA (2004) 101: 9897-9902).
Arabidopsis thaliana seedling development follows two distinct pathways: skotomorphogenesis or etiolation in the dark and photomorphogenesis or deetiolation in the light. The shift from skotomorphogenic to photomorphogenic development leads to a change in expression of approximately one-third of the total genes in Arabidopsis (Plant Cell (2001) 13, 2589-2607; Proc. Natl. Acad. Sci. (2001) 98, 9437-9442).
Regulation of transcription of specific genes is an important mechanism by which light regulates plant growth and development. CAB, RBCS, and CHS are well-studied genes that are upregulated by light. Investigations of the promoters of the light-inducible genes, including CAB, RBCS, and CHS, have led to identification of four commonly found light-responsive elements (LREs): G, GATA, GT1, and Z-box, which have been demonstrated to be essential for light-mediated transcriptional activity (Ann. Rev. Plant Physiol. Plant Mol. Biol. (1995) 46, 445-474; EMBO J. (1996) 15, 3732-3743; Plant J. (2002) 31, 741-753). Several LRE-specific transacting factors have been identified, and in some cases, their functions in light signaling pathways have been investigated (Semin. Cell Biol. (1994) 5, 335-346; Ann. Rev. Plant Physiol. Plant Mol. Biol. (1995) 46, 445-474; Plant Cell 9, 491-507).